From: Lee Donald (Lee.Donald@t-systems.co.uk)
Date: Wed Sep 14 2005 - 08:07:52 GMT-3
Tim,
Port-priority would be last in the decision process. End to end cost would
be first, if the costs are equal, then it would be based on the lower Bridge
ID. Lastly it would be port-ID, which consists of the port-priority and the
lowest port number.
That is why you would only use the port-priority on the root to a directly
connected switch, because the cost and Bridge-id would be identical down
each link, so port-priority would come into play.
Further down the network port-priority would never be used.
Regards
Lee.
-----Original Message-----
From: Tim [mailto:ccie2be@nyc.rr.com]
Sent: 14 September 2005 11:16
To: 'Anthony Sequeira'; 'Group Study'
Subject: RE: Load Balancing Across Trunks
Anthony,
I'd be very interested in seeing how you tested this.
If your topology is like this:
Root bridge --- sw1 ==== sw2
Where there are multiple physical paths between sw1 and sw2, there must be
someway that STP puts the redundant ports on sw2 into a blocked state and
determines which port will forward.
In your testing, what did you find was the way STP determined this?
Tim
-----Original Message-----
From: nobody@groupstudy.com [mailto:nobody@groupstudy.com] On Behalf Of
Anthony Sequeira
Sent: Wednesday, September 14, 2005 12:31 AM
To: Group Study
Subject: Re: Load Balancing Across Trunks
I have confirmed through lab tests that you cannot use port-priority on two
downstream switches from the root to control the choice of trunk port for
the traffic.
Certainly, as Tim suggested in this thread, if you use Root Guard to block
the backbone device - you can then get yourself in a situation where you can
use port-priority, since now you can control the election of the root
device.
I am waiting for someone to PROVE otherwise - but at this point - it looks
like load-balancing using port-priority is only an option when one of the
two switches you are trying to load balance between is the root!
On 9/13/05, Anthony Sequeira <terry.francona@gmail.com> wrote:
>
> It seems to be a simple task to load balance traffic on a VLAN basis
> across your trunk links if you are dealing with only two switches that you
> completely control. For example, if you are forbidden from using port
cost,
> just make one of your two switches the root for all VLANs and then set the
> port priorities apropriately on this upstream switch for each VLAN.
> But what if the root of a VLAN you need to load balance is on a third
> switch out of your control? Now you can play with port-priority all you
want
> on your two switches but your configurations will have no effect.
> Must we be able to control the root switch election in order to properly
> load balance across trunk links using port priority? I have "labbed" this
up
> - and it seems that we do need this level of control.
> Is there another way to control load balancing across trunk links beyond
> port cost and port priority? I think not.....
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